Hello, this may help with your quest. We calibrate around 60 Fluke 2626 sensors a year (Used with the 1620A). The 2626-S and -H sensor are exactly the same, they just use tighter calibration points. They both use the Honeywell HIH-4000 Sensor. Fluke does not pre-age these, however we have found after a year they are more stable (The Fluke manual references the initial calibration should be 2x in the first year to support this). When our contract started, we had some sensors that had not been calibrated for 10 years and they were only out of spec by <5%. From our experience after soldering, the Sensirion SHT45 and SHT85 require a bit more processing for stability (drying out and re-humidifying) vs the HIH-4000.
For calibration in field, we use a check/reference probe (HC2A-SH calibrated w/ Thunder Scientific PP Chamber) together with saturated salt solutions. We use Lithium Chloride, Potassium Acetate, Potassium Carbonate, and Sodium Chloride - All high purity ACS/Reagent grade mixed with DI water. Make sure to have more solute in the solution to maintain the equilibrium in case of excess moisture. As long as you are able to keep them at a fixed temp (We keep and use these in a temperature chamber or dry well calibrator) and allow 30min-1hr time to settle after inserting the probe this method has proven very stable and repeatable within <0.5%RH.
Refer to the NIST article
https://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf for your solution of choice.
We use Pyrex 1399 Bottles to keep the salt solutions. During calibration, we use the 195-45HTSC caps with a silicone septa that has had a hole punched through to seal the sensor. For odd sensors, we also have this silicone seal (DWK Kimble Silicone Lid) that can be put over - see photo.